1. Field of the Invention
The present invention relates to a technology for displaying a 3D map on which ground surfaces and features are represented three-dimensionally.
2. Description of the Related Art
Conventionally, a three-dimensional (3D) map on which features are represented three-dimensionally have been popular. Since the 3D map is easy to grasp a three-dimensional shape of a feature, it is highly convenient. This 3D map is drawn by projecting a 3D model of a feature from a view point set within a 3D space. In general, when a feature is to be drawn in a 3D map, in order to realistically display a depth feeling of the feature, depth determination (depth test) and hidden-surface processing are executed. Thus, a building that is hidden by other buildings and cannot be seen from the view point is not drawn. Moreover, an underground structure such as a tunnel is also hidden by a ground surface and is not drawn. As a result, in the 3D map, such obstructions can occur that a positional relation between the buildings cannot be understood easily or since the tunnel is not drawn, connection between roads cannot be understood easily or the like.
On the other hand, in a two-dimensional (2D) map, the building is not hidden by other buildings and as disclosed in Japanese Patent Laid-Open No. 9-138136, the tunnel is represented by a broken line or the like so that connection between the roads can be understood easily. That is, in the 3D map, by realizing realistic representation, to the contrary, convenience as a map is lost in a sense.
By considering these points, Japanese Patent Laid-Open No. 2004-333155 discloses a technology which makes it possible to visually recognize a point of interest by using transparent display of a building in front of the point of interest in the 3D map. Moreover, Japanese Patent Laid-Open Nos. 2008-128928 and 2003-166836 disclose a technology indicating a position of the underground structure on the map by drawing a planar shape of the underground structure on the ground surface.
However, when a map is drawn, the ground surface cannot be made transparent, and the technology in Japanese Patent Laid-Open No. 2004-333155 cannot be applied to representation of an underground structure. Applicable ranges of the technologies in Japanese Patent Laid-Open Nos. 2008-128928 and 2003-166836 are limited such that, if the ground surface has a 3D shape including irregularity, the technologies cannot be applied, and moreover, there remains a problem that they can only represent the position and the shape of the underground structure in a planar manner.